This invention relates to an image forming apparatus which forms images by superposing a photosensitive sheet with one surface coated with microcapsules encapsulating a colorless dye and a light-curable agent which hardens by exposure to light and an image transfer sheet with one surface coated with a developing agent having a chromogenic effect on this colorless dye and pressing them together.
Japanese Patent Publication Tokkai 58-88739 discloses a method of forming images by using a photosensitive sheet. The photosensitive sheet used by this method is characterized as having a sheet-like light-transmissive base material coated with microcapsules encapsulating a colorless dye of a specified kind and a light-curable agent which hardens when exposed to light. If a photosensitive sheet like this is exposed to image-forming light such as reflected light from a document, only those of the microcapsules on the sheet which are exposed to light are hardened and a latent image corresponding to the image carried by the light is thereby formed on the photosensitive sheet. If an image transfer sheet with one surface coated with a developing agent having a chromogenic effect on the colorless dye in the microcapsules is superposed on such a photosensitive sheet having a latent image formed thereon and if they are then pressed together, those of the microcapsules which have not been hardened rupture, causing the colorless dye encapsulated therein to flow out thereof and to react with the developing agent on the image transfer sheet to form a visible image corresponding to the latent image.
Examples of technologies for exposing a photosensitive sheet to image-forming light include the use of a liquid crystal panel in which an image has been formed. If light is made incident on such a panel and the photosensitive sheet is exposed to the transmitted or reflected light, a latent image can be formed as explained above by the hardened microcapsules. It is known that an image containing a large amount of information can be formed over a small area by using heat of a laser beam on an element such as a smectic liquid crystal panel in which image can be formed by an electro-thermo-optic effect. It is advantageous to make use of such a smectic liquid crystal panel with a small surface area to expose a photosensitive sheet to an image-forming beam of light because, unlike by prior art methods of exposure through a slit, the entire image on the liquid crystal panel can be projected onto the photosensitive sheet by means of a small optical system and it is possible to thereby speed up the process of forming images.
In order to rupture the microcapsules which have not been hardened on a photosensitive sheet described above, a high pressure on the order of 25-125 kg/cm is required and use is usually made of a compressing device with a pair of pressure rollers for applying such a high pressure. Such compression devices have been disclosed, for example, in U.S. Pat. No. 4,399,209 and Japanese Patent Publication Tokkai 62-6257.
Inconveniences are often encountered with image forming apparatus having such a compression device with a pair of pressure rollers because the rotations of the individual rollers may change suddenly when the front edge of an image transfer sheet (with thickness of 60-150.mu.m) superposing a photosensitive sheet is sent between the lines of contact on these rollers and when its back edge leaves the gap therebetween. Such sudden changes in the rotations of the rollers not only make the speed of feeding the image transfer sheet unstable but also cause vibrations in the optical system and the paper feeding system for the image transfer sheet, thereby adversely affecting the quality of the formed images with distortions and blurriness. There is also the problem of noise when an image transfer sheet enters or comes out of the gap between the pressure rollers. The noise of impact is particularly loud when an image transfer sheet is transported out of the gap.
Still more importantly, sudden changes in the rotations of the rollers tend to shorten the lifetimes of the various parts for driving the rollers such as gears and clutches. In order to prevent their lifetimes from becoming shortened, they must be either made of a stronger material or structurally so designed as to withstand changes in rotations. If such parts are used, however, use must also be made of a motor with a high output torque at the startup and this has the adverse effect of increasing the overall cost of the apparatus as well as its weight.